Thermosetting polyimides are easier to process than their thermoplastic counterparts in that they use low molecular weight, low viscosity monomers and/or prepolymers as starting materials in their synthesis. These thermosetting polyimides' superior processability combined with their high temperature capabilities make them increasingly more attractive for use as high performance matrix resins in typically lightweight, structurally efficient fiber reinforced polymer matrix composites. Such polymer matrix composites are finding increased use in, inter alia, the electronics, automobile and aerospace industries.
Addition-cured thermosetting polyimides are typically classified by the chemical nature of their reactive end-groups. Currently, there are at least three general types of polyimides, (1) PMR-type polyimides, (2) acetylene-terminated polyimides and (3) bismaleimides, although some polyimides may not fall under any of these types.
Bismaleimides are relatively easily processable using various molding techniques, including compression molding, autoclave molding and resin transfer molding, but do not possess the high temperature stability associated with the other two types of polyimides. Bismaleimides are popular, however, for use in the temperature range of 150-250.degree. because of their epoxy-like processing and polyimide-like temperature capability.
In comparison to the bismaleimides, acetylene-terminated polyimides exhibit higher glass transitions temperatures and increased thermo-oxidative stability. However, the acetylene-terminated polyimides are extremely difficult to process and, therefore, are not suitable for many applications due to the high cost involved during processing.
In 1972, an improved process, known as in-situ Polymerization of Monomer Reactants (PMR) for polyimide composite fabrication was developed by NASA. The PMR process essentially comprises dissolving a monoalkyl ester of 5-norbornene-2,3-dicarboxylic acid, also known as nadic ester (NE), an aromatic diamine, and a dialkyl ester of an aromatic tetracarboxylic acid in a low-boiling alkyl alcohol such as methanol or ethanol. The monomeric solution is used to impregnate other components such as reinforcing fibers, with in-situ polymerization through the nadic ester end group occurring directly on the fiber surfaces, producing a composite material with excellent thermal and mechanical properties. Attractive features of the PMR process include a) the use of low molecular weight, low viscosity monomers; b) the use of a low-boiling solvent; and c) little or no evolution of volatile materials during the final curing step. Thus, in comparison with the bismaleimides and the acetylene-terminated polyimides, the PMR-type polyimides are easy to process and exhibit high glass transition temperature and high temperature stability.
Unfortunately, these PMR-type polyimides are linear addition-cured polyimides which exhibit high melt flow viscosities of at least 100,000 centipoise that limit their processing to techniques involving hand lay-up of the prepreg followed by autoclave or compression molding. That is, resin transfer molding techniques cannot be used with these linear addition-cured polyimides. Use of these other processing techniques are extremely labor intensive and results in high manufacturing costs for components made with linear addition-cured polyimides. It will be appreciated that the term "linear" for the linear addition-cured polyimides refers to the general configuration of the polyimides as extending in only two directions.
Thus, the need exists for a PMR-type polyimide that exhibits a lower melt flow viscosity than known PMR-type polyimides. Such a PMR-type polyimide is believed to be more easily processable and, potentially, capable of being processed using low cost molding techniques such as resin transfer molding.
A significant benefit of using the PMR approach in the production of polyimides is that, for a given set of monomer reactants, a series of PMR-type polyimides can be formulated simply by changing the molar ratio of each monomer reactant. For example, the molecular weight of the typical linear PMR-type polyimide can be varied depending upon the ratio of the monomer reactants by selecting the molecular weight (n) of the diester diacid or the molecular weight of the diamine (n+1). The molar ratio of the nadic end group will always be 2 in these linear addition-cured polyimides. Thus, the ratio of end group:diamine: diester diacid will be 2:n+1:n. In theory, the formulated molecular weight of the polyimide will determine its crosslink density and is, therefore, thought to be an important parameter controlling the processing characteristics, physical and mechanical properties of a PMR-type polyimide.
Significant strides have been made in the production of polyimides having improved mechanical properties and high temperature stability and performance. For example, Takeichi and Stille have prepared biphenylene and acetylene end-capped imide oligomers by adjusting the stoichiometry of the monomers 3,3',4,4'-benzophenone tetracarboxylic dianhydride and 4,4'-diaminophenyl ether and the end-capping monomers (3- or 4-aminophenyl) acetylene or 2-aminobiphenylene. Star-branched imide oligomers having biphenylene or acetylene end groups were also prepared by utilizing 1,3,5-tris(4-aminophenoxy)benzene as the core of the star. Studies have shown that melt processed films of these star prepolymers exhibit better mechanical properties than films of their linear counterparts. For a more detailed account of these findings, see Takeichi, T. and Stille, J. K., Macromolecules, "Star and Linear Imide Oligomers Containing Reactive End Caps: Preparation and Thermal Properties", 19(8), 2093-2102, (1986), the disclosure of which is incorporated herein by reference. It will be appreciated, however, that Takeichi and Stille did not employ the PMR process and, thus, processability problems remain.
Similarly, strides have also been made in the synthesis of certain other addition-cured polyimides which improve various properties. For example, Jensen uses 4-phenylethynylphthalic anhydride as a reactive end cap and forms an addition-cured polyimide by dissolving n units of diamine, 2 units of triamine, namely triamino pyrimidine, n+l units of a dianhydride, namely 3,4'-oxydianiline, and 4 units of the phenylethynylphthalic anhydride reactive end group in N-methylpyrrolidinone (NMP). For a more detailed discussion on this reaction, see Jensen, B. J., Polym. Prepr., "Modified Phenylethynyl Terminated Polyimides with Lower Melt Viscosity" 37(2) 222-23 (1996), the disclosure of which is incorporated herein by reference. The polyimide suffers from the fact that upon imidization of one of the amine groups in the triamine, the reactivity of the other two amines is diminished substantially.
Moreover, it will be appreciated that, while the art has provided improvements to acetylene-terminated polyimides and to other "linear" polyimides, the art has not provided heretofore a manner in which to improve the processability of PMR-type polyimides to the extent that these polyimides would be suitable for processing using various molding techniques including resin transfer molding.